Retinoids are a class of compounds structurally related to vitamin A, comprising natural and synthetic compounds. Several series of retinoids have been found clinically useful in the treatment of dermatological and oncological diseases. Retinoic acid and its other naturally occurring retinoid analogs (9-cis retinoic acid, all-trans 3,4-didehydro retinoic acid, 4-oxo retinoic acid and retinol) are pleiotropic regulatory compounds that modulate the structure and function of a wide variety of inflammatory, immune and structural cells. They are important regulators of epithelial cell proliferation, differentiation and morphogenesis in lungs. Retinoids exert their biological effects through a series of hormone nuclear receptors that are ligand inducible transcription factors belonging to the steroid/thyroid receptor super family.
The retinoid receptors are classified into two families, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs), each consisting of three distinct subtypes (α, β, and γ). Each subtype of the RAR gene family encodes a variable number of isoforms arising from differential splicing of two primary RNA transcripts. All-trans retinoic acid is the physiological hormone for the retinoic acid receptors and binds with approximately equal affinity to all the three RAR subtypes, but does not bind to the RXR receptors for which 9-cis retinoic acid is the natural ligand. Retinoids have anti-inflammatory effects, alter the progression of epithelial cell differentiation, and inhibit stromal cell matrix production. These properties have led to the development of topical and systemic retinoid therapeutics for dermatological disorders such as psoriasis, acne, and hypertrophic cutaneous scars. Other applications include the control of acute promyelocytic leukemia, adeno- and squamous cell carcinoma, and hepatic fibrosis.
A limitation in the therapeutic use of retinoids has stemmed from the relative toxicity observed with the naturally occurring retinoids, all-trans retinoic acid and 9-cis retinoic acid. These natural ligands are non-selective in terms of RAR subtype and therefore have pleiotropic effects throughout the body, which are often toxic.
Various retinoids have been described that interact selectively or specifically with the RAR or RXR receptors or with specific subtypes (α, β, γ) within a class. RARA specific agonists have held high promise for the treatment of cancers and many have entered human clinical trials. However, only one RARA specific agonist, tamibarotene, has ever been approved for the treatment of cancer. Moreover, tamibarotene is only approved in Japan and only for the treatment of acute promyelocytic leukemia, despite trials in the US and Europe. The disconnect between the theoretical efficacy of RARA agonists in cancer and the dearth of regulatory approvals for such agents raises the question of why such agonists are not effective and safe in humans. Therefore, there is a need to better understand why RARA agonists have not met their therapeutic potential.
Recent advances in genomic technology and the understanding of gene regulatory circuits has led to the discovery of super enhancers. Whereas many genes in a given tissue or cancer type may be regulated by the presence of enhancers in proximity to the gene coding region, a small minority of these represent a highly asymmetric and disproportionately large loading of transcriptional marks and machinery relative to all other active genes. Recent discoveries suggest that such enhancers are tied to genes of special relevance to the function and survival of the cell harboring them. As such, an association of a super enhancer with a gene indicates the relative significance of said gene to the survival of that cell. These observations may be useful in predicting the efficacy of various therapies.